How to show children pages as array

You should use get_posts function.

global $post;
$parent_id = $post->ID;

$children = get_posts(array(
    'post_parent' => $parent_id,
    'fields' => 'ids'
));

$string = '<ul><li>' . implode('</li><li>', $children) . '</li></ul>';

• get_posts function
• WP_Query class